1. Field of the Invention
The present invention relates to a numerical controller for controlling the positions of servomotors or the positions of control axes controllably driven by the servomotors, and in particular to a numerical controller that initiates positional control in a novel way by supplying current to the servomotors in a state in which no current is supplied to the servomotors and the servomotors (movable parts) are coasting.
2. Description of the Related Art
In die casting machines, pressing machines, and other machines in which servomotors are used to drive movable molds (control axes) to open and close the molds, a large force is required to open the molds, so it is known that current is not supplied to the servomotors (this state will be referred to hereinafter as “servo-off”) and workpieces and the movable molds are ejected from the stationary molds by a hydraulic pressure or the like, and current is then supplied to the servomotors (this state will be referred to hereinafter as “servo-on”) to control the positions of the movable molds (control axes) that are coasting after the molds are opened.
A numerical controller that smoothly starts issuing move commands by setting the actual speed of an axis coasting in the servo-off state at the point in time the servo-off state is released and switched to the servo-on state as an initial speed of move command in positional control is proposed in a Japanese Patent Application (Serial No. JP2005-370821) and published on Jul. 15, 2007, as JP 2007-172394, A).
In die casting machines and pressing machines having large movable parts, synchronous control is adopted to control the activation of the large movable parts using two or more servomotors. In this case, when the workpiece and movable mold are ejected from the stationary mold with a hydraulic pressure or the like, the ejected points may slightly be displaced and the ejecting force may not be transmitted evenly to the two or more servomotors.
In the above Japanese patent application, when a plurality of servomotors are connected and operated synchronously in the numerical controller in which move commands are initiated when the servomotors, ejected by an external force and coasting in the servo-off state, are switched to the servo-on state, the actual speeds of the control axes at servo-on state are set as the initial speeds and identical move commands are subsequently initiated.
When a plurality of servomotors are connected and operated synchronously, setting the actual speeds at servo-on of the control axes as the initial speeds in the move commands and accelerating the axes at the same acceleration rates causes the difference in position between the control axes to gradually increase and problems to arise such as increasing loads applied to the connecting parts (see FIG. 20).